ABSTRACT Demand for primary care services is projected to grow considerably in the next decade. Non-physician clinicians, such as nurse practitioners (NPs) and physician assistants (PAs), may be able to help meet this growing demand. While PAs require physician supervision to practice, NPs can practice and prescribe independently in 23 states. In all other states, scope of practice (SOP) laws require NPs to practice with some degree of physician oversight. NP SOP is a topic of considerable debate. Proponents of expanded NP SOP argue it can increase access to high-quality, team-based care for patients, while relieving the pressure on physicians. Opponents of expanded NP SOP believe NPs are not adequately trained to treat complex patients. Expanded SOP for NPs has shown to improve some population health and access outcomes with minimal effects on expenditures. However, data availability has limited researchers? ability to examine empirically the relationship between SOP and process of care measures in primary care, including how practices reorganize when SOP expands and whether care quality changes. Ten states expanded NP SOP between 2011 and 2017. In this study, I will rely on within-state and over- time variation in SOP state laws to evaluate the effect of expanded NP SOP on: 1) the division of labor between physicians and NPs in primary care practices (AIM 1); 2) the clinical complexity of patients seen by physicians and NPs (AIM 2); and 3) rates of inappropriate antibiotics prescribing ? a measure of poor-quality care (AIM 3). I will use a quasi-experimental ?difference-in-differences? research design, comparing outcomes in states that expanded NP SOP to those that did not, before and after the legislative changes. Previous research on NP SOP has relied largely on single-payer claims or survey data, which have major limitations, including: 1) they are unable to account for ?incident to? billing (when NP-rendered services are billed under physician identifiers); 2) they may reflect self-reported outcomes; and/or 3) they lack rich information on clinician behavior. To overcome these limitations, I will use a relatively new, national dataset of electronic health records linked to claims from all payer types. These data contain three crucial elements: 1) identifiers for rendering and supervising clinicians, rather than only identifiers for billing clinicians; 2) time stamps and details about practice organization and staffing (i.e. who worked where, when, and with whom); 3) detailed information on all orders (e.g. prescriptions) placed. This project has particular relevance for AHRQ priority populations, including patients in primary care shortage areas or underserved, rural communities. In these areas, increased independence for NPs has the potential to improve access to care and facilitate team-based primary care delivery. My research question ? how does expanding NP SOP affect the organization of primary care practices and quality of care? ? will inform policymakers as they respond to ongoing health care delivery system and workforce challenges.